Methods of preparation of hydrocarbon derivatives of lithium compounds have been published by various workers in the field, such as, for example, K. E. Eberly, U.S. Pat. No. 2,947,793, which issued Aug. 2, 1960, which patent teaches a process for preparation of alkylene dilithium compounds.
Eberly, in U.S. Pat. No. 3,122,592, discloses a method for the preparation of alkyl lithium compounds which involves reacting a monohaloalkane comprising 3 to 8 carbon atoms with finely divided particles of an alloy consisting of lithium and from 0.3 to 1.0 percent sodium or potassium, the reaction being effected at a temperature of from 0-60° C. Eberly discloses that the yield of carbon-bonded lithium increases to a maximum of 87.89 percent at a corresponding level of 0.36 percent sodium alloyed in lithium. A drawback to the process is that the reaction is slow, with the addition of monohaloalkane requiring up to several hours, followed by additional time to allow the reaction to become complete and potentially several hours standing time to allow separation of products and by-products.
A process for producing alkyllithium containing six or more carbon atoms, such as octyllithium is disclosed by C. Guo and coworkers, J. Am. Chem. Soc., 1985, 107, 6030 who employed a refluxing hexane medium and a four hour post addition reflux to obtain a yield of about 70%.
Similarly, U.S. Pat. No.3,452,112 to Kamienski et al. discloses a method of preparing organic solvent solutions of lithium-hydrocarbon compounds which comprises adding a preformed alkyl lithium compound to a dispersion of finely divided lithium in a non-reactive liquid medium, and subsequently adding to this mixture an unsaturated hydrocarbon or a hydrocarbon halide which is reactive with lithium to produce the desired product. Kamienski further discloses that the reaction is carried out at a temperature of between −50 to 5 ° C., and that the lithium metal used be essentially pure or commercially available material, employing a small amount of sodium metal of about 0.25 to about 1 weight percent based on the lithium metal. The patent discloses, however, that the addition of the reactants and the time taken to bring the reaction to completion could take several hours.
High purity concentrated alkyllithium solutions are highly desirable as it is important that the alkyllithium product be free from or at least low in olefin content as olefins lead to the development of deep yellow colored alkyllithium products. The level of chloride ion impurities is important as high chloride values of 300 ppm and above generally result in hazy alkyllithium products. While concentrated, clear solutions of alkyllithium compounds in hydrocarbon solvents are clearly desirable, they are difficult to obtain. One reason for this is that concentrated solutions of alkyllithium compounds are highly viscous so that unreacted excess lithium generally employed in the reaction is difficult to remove by filtration or other conventional particle separation means. The prior art processes produce alkyllithium compounds containing over 300 parts per million of dissolved inorganic halides, including, inter alia, lithium chloride, which are not removed by filtration. When the resulting product-containing solutions are concentrated by distillation or other solvent removal means, by-products such as lithium halide form fine crystals in the product solution. This leads to a hazy appearance in the product, and is generally undesirable in industrial processes. Further, the halide by-products mat actually precipitate out of solution, resulting in an even more undesirable product. Thus, a process which leads to the production of alkyllithium compounds in high yields of 90% or better, are highly desirable economically.
U.S. Pat. No. 3,452,112, discussed above, results in product alkyllithium solutions in yields lower than 85 percent. Unsaturated hydrocarbon-lithium adducts are produced in higher yields, but are colored solutions.
U.S. Pat. No. 5,332,533 discloses a process for producing alkyllithium by reacting a primary alkyl halide with lithium metal in a liquid hydrocarbon solvent in an inert atmosphere at a temperature between 35 and 125 degrees centigrade. This process, however, also requires an extended feed time followed by further time to bring the reaction to completion. Further, the products of the reactions are generally solutions having a yellow color. Therefore, a need remains for a process for preparation of clear, colorless solutions of alkyllithium compounds which forms such products in high yield and high purity and in shorter reaction times. The present invention overcomes the disadvantages of the prior art and provides a process that satisfies this need.